ATP-dependent potassium (K.sub.ATP) channels serve to couple metabolic state to electrical activity in many types of cells. By hyperpolarizing the cell, K.sub.ATP channels limit electrical activity and hence reduce Ca.sup.2+ entry into muscle and nerve cells. In the pancreas, they are a critical link between blood glucose concentration and insulin secretion.
Sulfonylureas (SUs) are oral hypoglycemics widely used in the treatment of non-insulin dependent diabetes mellitus (NIDDM). SUs stimulate insulin release from pancreatic islet .beta. cells. The mechanism for insulin release involves 1) inhibition of a K.sub.ATP channel which sets the .beta. cell resting membrane potential, 2) reduction of K.sup.+ outflow which causes .beta. cell depolarization and 3) the activation of one or more voltage-dependent L-type calcium channels which results in Ca.sup.2+ influx, exocytosis, and insulin release. SUs such as tolbutamide or glyburide decrease K.sub.ATP channel activity, thereby depolarizing the cell and triggering insulin release.
Until recently the K.sub.ATP channel and the sulfonylurea receptor (SUR) were thought to be the same molecule (Aguilar-Bryan et al (1995) Science 268:423-426); however, SUR does not possess intrinsic K.sup.+ channel activity (Ammala C et al (1996) Nature 379:545-548). Instead SUR interacts with inward-rectifier K.sup.+ channels, conferring SU and ATP sensitivity to and modulating the activity of these channels (Inagaki N et al (1995) Science 270: 1166-1170).
A second isoform of SUR, denoted SUR2, has recently been discovered in rat. This isoform has different tissue distribution and different SU and ATP binding properties from rat SUR (Inagaki N et al (1996) Neuron 16:1011-1017). The channel kinetics of Kir6.2, an inward-rectifier K.sup.+ channel, co-expressed with SUR2 are different than the channel kinetics of Kir6.2 co-expressed with SUR. Based on these observations, it is suggested that a family of structurally related but functionally distinct SURs determine the ATP sensitivity and pharmacological responses of K.sub.ATP channels in various tissues (Inagaki N et al (1996), supra).
SURs from rat and hamster consist of 1581 and 1582 amino acids, respectively, with 12 potential membrane-spanning helices (Aguilar-Bryan et al, supra). In addition, the proteins contain two domains having strong similarity to the nucleotide binding folds (NBFs) of the ATP-binding cassette (ABC) superfamily of proteins. The proposed topology of the rat, hamster, and a recently reported human SUR (GenBank GI 1369844; unpublished) consists of an external amino terminus, nine predicted transmembrane helices, the first cytosolic NBF (NBF-1), four more transmembrane helices, the second cytosolic NBF (NBF-2) and a cytosolic C-terminus. The topology of the SURs are similar to other members of the ABC superfamily including multidrug resistance (MDR) proteins and cystic fibrosis transmembrane regulators (CFTR; Philipson LH and Steiner DF (1995) Science 268:372-373).
The NBFs of ABC superfamily proteins control activity through their interaction with cytosolic nucleotides. In cystic fibrosis, the more frequent and severe disease mutations are located in the nucleotides encoding the two NBFs of the CFTR protein (Tsui L-C (1992) Trends Genet 8:392). Familial persistent hyperinsulinemic hypoglycemia of infancy (PHHI) may be caused by mutations affecting NBF-2 of SUR (Thomas P M et al (1995) Science 268:426-429).
SU-sensitive K.sub.ATP channels are present in brain cells and play a role in neurosecretion at nerve terminals. K.sub.ATP channels in the substantia nigra, a brain region that shows high SU binding, are inhibited by high glucose concentrations and antidiabetic SUs, and are activated by ATP depletion and anoxia. Furthermore, inhibition of the K.sub.ATP channel activates gamma-aminobutyric acid (GABA) release, whereas K.sub.ATP channel activation inhibits GABA release (Amoroso S et al (1990) Science 247:852-854; Schmidt-Antomarchi et al (1990) Proc Natl Acad Sci USA 87: 3489-3492).
Action potentials in cardiac cells are modulated by SU compounds binding to SURs. The duration of the action potential of guinea pig cardiac cells was drastically reduced by decreasing intracellular ATP concentrations ([ATP].sub.in) by perfusion or by blockade of oxidative phosphorylation. Glibenclamide, an SU compound, was found to restore normal or nearly normal action potentials in these [ATP].sub.in -depleted cardiac cells. (Fosset M et al (1988) J Biol Chem 263:7933-7936). Restoration was attributed to inhibition of cardiac K.sub.ATP channels by sulfonylurea compounds acting via the SURs.
SURs confer ATP and SU sensitivity to inwardly-rectifying potassium channels, thereby coupling metabolic state to electrical activity in tissues such as brain, pancreas, and heart. SURs are useful in the diagnosis and treatment of diseases related to abnormal K.sub.ATP channel function, such as NIDDM and PHHI. The selective modulation of the expression or activities of SURs may allow the successful management of such diseases.